OBJECTIVE: To compare adolescent mothers living in four areas with different degrees of social exclusion in the city of Santo André, São Paulo, Brazil, in terms of the mothers' schooling, the birth weight and gestational age of their babies, and the specific fertility rate of each of the four areas in 1998. METHOD: An ecological cross-sectional study was carried out with 1 314 adolescent girls. The four areas analyzed had earlier been defined in the City of Santo André Social Exclusion/Inclusion Map. Area 1 had the highest exclusion index (worst socioeconomic conditions), and Area 4 the lowest exclusion index (best socioeconomic conditions). The data relating to the adolescent mothers and their children were collected from the National Live Birth Information System, and the socioeconomic data for Santo André were obtained from the State Data Analysis System Foundation, the Brazilian Institute of Geography and Statistics, and the city's Social Exclusion/ Inclusion Map. RESULTS: Having little formal education was statistically associated with the poorest areas. Of the infants with a birthweight < 2 500 g, 76.8% of them were born in the two poorest areas of the city. The highest fertility rate (35.7 per 1 000 adolescents) was found in Area 1, the area with the worst socioeconomic conditions; the lowest fertility rate (12.1 per 1 000) was found in Area 4, the area with the best socioeconomic conditions. The proportion of births that were premature did not differ among the four areas (P = 0.81). CONCLUSIONS: The results showed that adolescent girls with little schooling and a lower socioeconomic level were more likely to give birth. Specific actions should be promoted to prevent pregnancy in this group and to foster the social inclusion of these adolescents and their children, providing them with opportunities to improve their socioeconomic situation.